// Copyright 2016 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_TASK_SCHEDULER_SEQUENCE_H_
#define BASE_TASK_SCHEDULER_SEQUENCE_H_

#include <stddef.h>

#include <memory>
#include <queue>

#include "base/base_export.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/task_scheduler/scheduler_lock.h"
#include "base/task_scheduler/sequence_sort_key.h"
#include "base/task_scheduler/task.h"
#include "base/task_scheduler/task_traits.h"

namespace base {
namespace internal {

    // A sequence holds tasks that must be executed in posting order.
    //
    // Note: there is a known refcounted-ownership cycle in the Scheduler
    // architecture: Sequence -> Task -> TaskRunner -> Sequence -> ...
    // This is okay so long as the other owners of Sequence (PriorityQueue and
    // SchedulerWorker in alternation and
    // SchedulerWorkerPoolImpl::SchedulerWorkerDelegateImpl::GetWork()
    // temporarily) keep running it (and taking Tasks from it as a result). A
    // dangling reference cycle would only occur should they release their reference
    // to it while it's not empty. In other words, it is only correct for them to
    // release it after PopTask() returns false to indicate it was made empty by
    // that call (in which case the next PushTask() will return true to indicate to
    // the caller that the Sequence should be re-enqueued for execution).
    //
    // This class is thread-safe.
    class BASE_EXPORT Sequence : public RefCountedThreadSafe<Sequence> {
    public:
        Sequence();

        // Adds |task| at the end of the sequence's queue. Returns true if the
        // sequence was empty before this operation.
        bool PushTask(std::unique_ptr<Task> task);

        // Returns the task in front of the sequence's queue, if any.
        const Task* PeekTask() const;

        // Removes the task in front of the sequence's queue. Returns true if the
        // sequence is empty after this operation. Cannot be called on an empty
        // sequence.
        bool PopTask();

        // Returns a SequenceSortKey representing the priority of the sequence. Cannot
        // be called on an empty sequence.
        SequenceSortKey GetSortKey() const;

    private:
        friend class RefCountedThreadSafe<Sequence>;
        ~Sequence();

        // Synchronizes access to all members.
        mutable SchedulerLock lock_;

        // Queue of tasks to execute.
        std::queue<std::unique_ptr<Task>> queue_;

        // Number of tasks contained in the sequence for each priority.
        size_t num_tasks_per_priority_[static_cast<int>(TaskPriority::HIGHEST) + 1] = {};

        DISALLOW_COPY_AND_ASSIGN(Sequence);
    };

} // namespace internal
} // namespace base

#endif // BASE_TASK_SCHEDULER_SEQUENCE_H_
